Which C.S. Lewis book collects letters written by a senior demon to his nephew?

The C.S. Lewis book collecting letters from a senior demon to his nephew is The Screwtape Letters.

The Screwtape Letters was published in book form in 1942. It consists of letters from Screwtape, an experienced demon, to his junior nephew Wormwood, who is trying to influence a human being. Lewis uses the reversed viewpoint to satirize temptation, self-deception, pride, distraction, and other moral weaknesses.

The book is fiction, but it is written as Christian apologetic literature rather than as a conventional supernatural adventure. Its epistolary structure means that the story unfolds through correspondence instead of through a narrator describing events directly.

The letters first appeared during the Second World War, when Lewis was also giving radio talks that later contributed to Mere Christianity. The demon narrator is Screwtape; Wormwood is the inexperienced nephew, so the two names should not be swapped.